Superhigh capacity and rate capability of high-level nitrogen-doped graphene sheets as anode materials for lithium-ion batteries

Electrochimica Acta 90 (2013) 492–497

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Superhigh capacity and rate capability of high-level nitrogen-doped graphene sheets as anode materials for lithium-ion batteries Dandan Cai a , Suqing Wang a , Peichao Lian b , Xuefeng Zhu c , Dongdong Li a , Weishen Yang c , Haihui Wang a,∗ a

School of Chemistry and Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou, China Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming, China c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China b

a r t i c l e

i n f o

Article history: Received 27 September 2012 Received in revised form 23 November 2012 Accepted 26 November 2012 Available online 16 December 2012 Keywords: Nitrogen-doped Graphene sheets Anode material Lithium-ion batteries

a b s t r a c t A new facile approach is proposed to synthesize nitrogen-doped graphene sheets with the nitrogendoping level as high as 7.04 at.% by thermal annealing pristine graphene sheets and low-cost industrial material melamine. The high-level nitrogen-doped graphene sheets exhibit a superhigh initial reversible capacity of 1123 mAh g−1 at a current density of 50 mA g−1 . More significantly, even at an extremely high current density of 20 A g−1 , highly stable capacity of about 241 mAh g−1 could still be obtained. Such an electrochemical performance is superior to those previously reported nitrogen-doped graphene sheets. The excellent electrochemical performance can be attributed to the two-dimensional structure, disordered surface morphology, high nitrogen-doping level, and the existence of pyridinic nitrogen atoms. The results indicate that the high-level nitrogen-doped graphene sheets could be a promising anode material for high-performance lithium-ion batteries. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Lithium-ion batteries play an important role in portable electronic devices because of their high energy density, long cycling lifetime and excellent safety. In recent years, they have also been proposed for being applied in hybrid electric vehicles and electric vehicles, which promoted the great research interests in developing lithium-ion batteries with high reversible capacity, excellent rate performance and cycling stability [1,2]. Graphite, the commercial anode material, also encounters some disadvantages such as low theoretical specific capacity (372 mAh g−1 ) and poor rate performance [3]. Thus, it is imperative to develop new high-performance electrode materials for lithium-ion batteries. Graphene has been considered to be a promising anode material for lithium-ion batteries and exhibits a large reversible specific capacity due to its large surface area, superior electronic conductivity and high chemical stability [4–10]. In addition, the electronic and chemical properties of graphene can be modified by chemical doping heteroatoms, such as boron atoms and nitrogen atoms [11–15]. Particularly, nitrogen-doped graphene has been extensively applied in various fields, such as electrocatalysis [16,17], supercapacitor [18,19], Li-air fuel cell [20], and lithium-ion

∗ Corresponding author. Tel.: +86 20 87110131; fax: +86 20 87110131. E-mail address: [email protected] (H. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.11.105

batteries [21–24]. Recently, nitrogen-doped graphene has been widely investigated and delivers better electrochemical performance than the pristine graphene [21–24]. The high nitrogendoping level could help improve the rate capability and reversible capacity of the nitrogen-doped graphene sheets because of the more disorder degree induced [12,23,25]. The pyridinic nitrogen atoms could also be beneficial to increase the reversible capacity of the nitrogen-doped graphene electrode, which could be attributed to the stronger electronegativity of nitrogen than that of carbon [20,25–27]. However, the synthetic methods of nitrogen-doped graphene are mainly limited to chemical vapor deposition (CVD) [20], thermal annealing of graphene oxide with NH3 [28], nitrogen plasma treatment of graphene [29], solvothermal synthesis [30], and the arc-discharge method [31]. The CVD method cannot meet the demand of large scale production of nitrogen-doped graphene [21]. Besides, nitrogen-doping level of the nitrogendoped graphene prepared by NH3 gas thermal treatment method is low (2–4%), and NH3 is a corrosive gas. Additionally, the last three methods are complicated and high cost. Therefore, it is still a challenge to prepare nitrogen-doped graphene sheets with high nitrogen doping level by a facile method. Herein, we propose a simple and facile approach to synthesize high-level nitrogen-doped graphene sheets by thermal annealing pristine graphene sheets and low-cost industrial material melamine. The electrochemical performance of the obtained material was investigated, and compared with those previously

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Fig. 1. The TEM images of (a) the graphene sheets and (b) the nitrogen-doped graphene sheets.

reported nitrogen-doped graphene sheets. The as-prepared nitrogen-doped graphene sheets as anode materials for lithiumion batteries show a superhigh specific capacity and excellent rate performance. 2. Experimental 2.1. Materials preparation Nitrogen-doped graphene sheets were prepared via a simple thermal annealing approach involving pristine graphene sheets and low-cost industrial material melamine. First, the pristine graphene sheets were prepared by rapid thermal expansion method according to our previous report [10]. Then, the pristine graphene sheets and melamine (Tianjin Kermel Chemical Reagent Co., Ltd., China) were mixed together with a weight ratio of 1:6 by grinding for 2 h. The mixture in a combustion boat was then placed in the center of a quartz tube in argon gas atmosphere and heated to 700 ◦ C at a heating rate of 5 ◦ C/min. After the temperature was maintained for 1 h, the furnace was cooled to room temperature slowly. The final product was taken out of the combustion boat. For comparison, the graphene sheets were prepared under the same thermal treatment condition without adding melamine. In addition, the bare melamine powder was also annealed in the combustion boat under the same condition without adding the pristine graphene sheets. After the reaction was finished, carbonized melamine was found and collected in the inner wall of mouth of the quartz tube. 2.2. Characterization of materials The structure and morphology were characterized by transmission electron microscopy (TEM) (FEI, Tecnai G2 F30 S-Twin). Raman spectra were measured using a Horiba Jobin Yvon LabRam Aramis Raman spectrometer with a laser of 632.8 nm. X-ray photoelectron spectroscopy (XPS) analysis was carried out using an ESCALAB 250 spectrometer (Thermo Fisher Scientific) with the mono Al K␣ radiation (1486.6 eV) under a pressure of 2 × 10−9 Torr. Elemental analysis was carried out with pure oxygen combustion method (vario EL III elementar, Germany). The nitrogen content in the nitrogen-doped graphene sheets was obtained by burning the sample to form oxynitride, which can be detected by a thermal conductivity detector (TCD).

2.3. Electrochemical measurements The electrochemical performances of the nitrogen-doped graphene sheets and the graphene sheets were investigated using coin cells (CR2025). The slurry contained the active materials (the nitrogen-doped graphene sheets or the graphene sheets) (75 wt.%), Super P (15 wt.%), and poly(vinylidene fluoride) (PVDF, Kureha, Japan) binder (10 wt.%) in an N-methyle-2-pyrrolidone (NMP, Tianjin Kermel Chemical Reagent Co., Ltd., China) solvent was pasted on a copper foil. Highly pure lithium foil was used as the counter electrode. Meanwhile, the celgard 2325 membrane was used as the separator. The electrolyte was composed of 1 mol L−1 LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethylcarbonate (DEC) (1:1 by volume) (Beijing Institute of Chemical Reagents, China). The coin cells were assembled in an argon-filled glove box (Mikrouna, super 1220) where the oxygen and moisture contents were less than 1 ppm. The cells were galvanostatically discharged and charged using a Battery Testing System (Neware Electronic Co., China) between 0.01 and 3.0 V. 3. Results and discussion 3.1. Microstructural characterization Fig. 1 shows representative TEM images of the graphene sheets and the nitrogen-doped graphene sheets. It can be clearly observed that the nitrogen-doped graphene sheets still maintain the twodimensional ultrathin flexible structure of the graphene sheets [4,5]. However, nitrogen-doped graphene sheets have more corrugations and scrolling than the graphene sheets, which is attributed to the defective structures formed by chemical doping nitrogen atoms [16,23]. The disordered structures in the nitrogen-doped graphene sheets could increase lithium storage sites, which is well consistent with the results of electrochemical performance. Raman spectra of the nitrogen-doped graphene sheets and the graphene sheets are shown in Fig. 2. The Raman spectrum of the nitrogen-doped graphene sheets displays two dominant peaks at about 1326 and 1590 cm−1 corresponding to the D band and G band, respectively. Moreover, the G band of the nitrogen-doped graphene sheets is down-shifted to 1590 cm−1 compared with the graphene sheets (1591 cm−1 ). The downshift of the G band can be attributed to electron-donating capability of nitrogen heteroatoms

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a

Survey

C1s N-doped graphene sheets N1s

Intensity / a.u.

1326

1590

G

a

1324

O1s

Intensity / a.u.

D

1591

Carbonized melamine

Graphene sheets

b

1400

Raman Shift / cm-1

1600

0

1800

200

400

600

800

1000

Binding Energy / eV

Fig. 2. Raman spectra of (a) the nitrogen-doped graphene sheets and (b) the graphene sheets.

[16,17,32,33]. As is known, the G band with E2g symmetry is related to phonon vibrations in sp2 carbon materials, while the D band with A1g symmetry is ascribed to disordered carbon, edge defects, and other defects [32,33]. Generally, the intensity ratio of D band to G band (ID /IG ) is used to estimate the disorder degree of graphene. As shown in Fig. 2, the nitrogen-doped graphene sheets show apparently higher ID /IG (1.29) than the graphene sheets (0.86), suggesting that the nitrogen-doping process generates extrinsic defects on the graphene surface [34]. Table 1 shows carbon, oxygen and nitrogen content of the samples obtained by XPS. It can be seen from Table 1 that the oxygen content of the pristine graphene sheets is 9.47 at.%, which is much higher than that of the nitrogen-doped graphene sheets (4.21 at.%). We also tested the oxygen content of graphene sheets by XPS. After high temperature thermal treatment, the oxygen content of graphene sheets is decreased to 5.68 at.%, indicating that the pristine graphene sheets were partly reduced under a high temperature. The phenomenon is consistent with the previously reported [8,22]. The difference of the oxygen contents of the three samples shows nitrogen-doping and reduction of residual oxygen of the pristine graphene sheets was achieved simultaneously in the thermal annealing process [16,19,35,36]. The binding energies in the XPS analysis were corrected with reference to the C 1s peak (284.8 eV). In Fig. 3a, the appearance of N 1s peak in the spectrum indicates that nitrogen was successfully doped into graphene sheets for the nitrogen-doped graphene sheets. Moreover, an atomic percentage of doped nitrogen is about 7.04 at.% based on XPS, which is higher than that of the previously reported for lithium-ion batteries [22–24]. For example, Sun’s group [22] synthesized nitrogen-doped graphene nanosheets by annealing graphene sheets in the presence of ammonia gas (NH3 ) and the nanosheets contain 2.8 at.% nitrogen. Moreover, Cheng and his coworkers [23] also made the nitrogen-doped graphene with nitrogen-doping level of 3.06% by thermal treatment of the pristine graphene in mixed gas atmosphere of NH3 and argon, and found that the rate capability of the doped graphene at high current

b

C 1s

N-doped graphene sheets

284.8

Intensity / a.u.

1200

285.3

286.6

280

282

284

286

288

290

292

Binding Energy / eV

c

N-doped graphene sheets

N 1s 398.4

Intensity / a.u.

1000

Pristine graphene sheets

400.9 399.6

394

396

398

400

402

404

406

Binding Energy / eV Fig. 3. (a) The XPS spectra of the nitrogen-doped graphene sheets and carbonized melamine; (b) C 1s XPS spectra and (c) N 1s XPS spectra of the nitrogen-doped graphene sheets.

Table 1 Carbon, oxygen and nitrogen content (at.%) of the samples obtained by XPS. Samples

C

O

N

Pristine graphene sheets Nitrogen-doped graphene sheets Graphene sheets Carbonized melamine

90.53 88.75 94.32 41.06

9.47 4.21 5.68 4.39

– 7.04 – 54.55

rates can be further improved with increasing doping concentration in graphene. Cui’s group [24] also synthesized nitrogen-doped graphene nanosheets with a doping level of ca. 2% nitrogen. It should be noted that the melamine plays an important role in the forming process of high-level nitrogen-doped graphene sheets. It

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The first five discharge/charge profiles of the nitrogen-doped graphene sheets and the graphene sheets at the current density of 50 mA g−1 are shown in Fig. 4a and b, respectively. Both the nitrogen-doped graphene sheets and the graphene sheets have the similar charge/discharge curves [22–24]. The presence of the plateau at about 0.9 V in the first cycle could be attributed to the formation of solid electrolyte interface (SEI) film on the surface of the graphene sheets [10,23,38,39]. The first reversible specific capacity of nitrogen-doped graphene sheets is as high as 1123 mAh g−1 , which is much higher than that of the graphene sheets (around 848 mAh g−1 ). Moreover, the high-level nitrogen-doped graphene sheets possess higher reversible specific capacity than those previously reported nitrogen-doped graphene sheets (684 mAh g−1 after 501 cycles [22], 1040 mAh g−1 at a low rate of 50 mA g−1 [23], and 900 mAh g−1 at a low current density of 42 mA g−1 [24]). The reasons for the superhigh reversible capacity are likely to be the large number of surface defects, presence of the pyridicnic N, and highlevel nitrogen doping [12,22,24–26,40]. Notably, the charge curves of the nitrogen-doped graphene sheets were almost consistent in the subsequent cycles, which could be attributed to the more stable SEI film forming during the first discharge process in the nitrogendoped graphene sheets. Fig. 5 presents the reversible charge/discharge capacity versus cycle number of the nitrogen-doped graphene sheets and the graphene sheets at the current density of 50 mA g−1 between 0.01 and 3.0 V. The nitrogen-doped graphene sheets exhibit better cycling stability and higher specific capacities than the graphene sheets. The reversible capacities of the nitrogen-doped graphene sheets are about 1123 mAh g−1 in the first cycle and 1136 mAh g−1 after 50 cycles. For the graphene sheets, the reversible capacities only are 848 mAh g−1 in the first cycle and 741 mAh g−1 after 50 cycles. The increase in the reversible capacities of the nitrogendoped graphene sheets over the graphene sheets could be ascribed to the topological defects on the graphene sheets [21,23]. Fig. 6 shows the rate capabilities and cycle performance of nitrogen-doped graphene sheets and the graphene sheets electrodes at various current densities from 0.5 to 20 A g−1 . At a current density of 0.5 A g−1 , the nitrogen-doped graphene sheets can be reversibly charged to 608 mAh g−1 , while the corresponding value of the graphene sheets is only 445 mAh g−1 . More significantly, even

Potential vs. (Li/Li+) / V

3.0

discharge 1 st charge 1

2.5

discharge 2 nd charge 2

2.0 discharge 3 rd charge 3

1.5 1.0

discharge 4 th charge 4

0.5

discharge 5 th charge 5

st

nd

rd

th

th

0.0 0

500

1000

1500

2000

2500

3000

Specific Capacity / mAh g-1

b 3.0

Potential vs. (Li/Li+) / V

3.2. Electrochemical properties of the nitrogen-doped graphene sheets and the graphene sheets

a

discharge 1 st charge 1

2.5

discharge 2 nd charge 2

2.0

discharge 3 rd charge 3

1.5 1.0

discharge 4 th charge 4

0.5

discharge 5 th charge 5

st

nd

rd

th

th

0.0 0

400

800

1200

1600

2000

Specific Capacity / mAh g-1 Fig. 4. First five discharge/charge profiles of (a) the nitrogen-doped graphene sheets and (b) the graphene sheets at the current density of 50 mA g−1 .

at an extremely high current density of 20 A g−1 , high stable capacity of about 241 mAh g−1 could still be obtained in nitrogen-doped graphene sheets, which is higher than the value of the graphene sheets electrode (about 112 mAh g−1 ) and the previously reported nitrogen-doped graphene sheets (199 mA g−1 at 25 A g−1 ) [23].

2600

Specific Capacity / mAh g -1

can be seen from Fig. 3a and Table 1 that the nitrogen content of the carbonized melamine is as high as 54.55 at.%. The high nitrogen content could provide enough nitrogen atoms in the forming process of nitrogen-doped graphene sheets. According to the above results, the possible nitrogen doping mechanism was discussed. The oxygen-containing groups of the pristine graphene sheets were partly removed at high temperature and as a result formed active sites for nitrogen doping into graphene framework. Meanwhile, nitrogen atoms or other nitrogen species formed by decomposition of melamine can attack these active sites and form nitrogen-doped graphene sheets [16,36]. As shown in Fig. 3b, the main peak at 284.8 eV is related to the graphite-like sp2 C, which indicates that most of the C atoms in the nitrogen-doped graphene sheets are arranged in a conjugated honeycomb lattice. The two weak peaks located at 285.3 and 286.6 eV represent N C (sp2 ) and N C (sp3 ) bonds, respectively [23]. Similarly, the N 1s peak can be divided into three components centered at 398.4, 399.6, and 400.9 eV, representing pyridine, pyrrolic, and graphitic nitrogen atoms in the nitrogen-doped graphene sheets, respectively [16,17,21–24]. From Fig. 3c, the pyridine nitrogen atom has been found to be the main form of nitrogen in the asprepared nitrogen-doped graphene sheets [16,37].

495

N-doped graphene sheets charge N-doped graphene sheets discharge Graphene sheets charge Graphene sheets discharge

2400 2200 2000

1200 900 600 300 0 0

10

20

30

40

50

Cycle Number Fig. 5. Reversible charge/discharge capacity versus cycle number of the nitrogendoped graphene sheets and the graphene sheets at the current density of 50 mA g−1 between 0.01 and 3.0 V.

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1700

N-doped graphene sheets charge N-doped graphene sheets discharge Graphene sheets charge Graphene sheets discharge

Specific Capacity / mAh g -1

1600 1500 1400 1300

-1 0.5 A g

-1 0.5 A g

700 600 500

-1 -1 5Ag 10 A g

300

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (no. 20936001), the Cooperation Project in Industry, Education and Research of Guangdong Province and Ministry of Education of China (no. 2010B090400518) and the Fundamental Research Funds for the Central Universities, SCUT (2009220038).

-1 1Ag

400

that the high-level nitrogen-doped graphene sheets are promising candidates as anode materials for high-performance lithium-ion batteries.

-1 15 A g 20 A g-1

200 100 0 0

10

20

30

40

50

60

70

Cycle Number Fig. 6. Rate capabilities and cycle performance of nitrogen-doped graphene sheets and the graphene sheets electrodes at various current densities, from 0.5 to 20 A g−1 .

Fig. 7. A schematic model of lithium-ion storage in defects.

Additionally, when the current density is tuned back to 0.5 A g−1 after cycling at different rates, the specific capacities could increase back to 674 mAh g−1 , which is higher than the value of the first ten cycles at a current density of 0.5 A g−1 . The increased capacity with cycling could be attributed to the activation process of the high-level nitrogen doping anode materials [12]. The more excellent rate capability of nitrogen-doped graphene sheets over the graphene sheets can be attributed to the following reasons. Firstly, nitrogen-doping could induce a lot of topological defects on the graphene sheets surface and increase disorder degree of graphene sheets, which further increase lithium storage sites of graphene sheets (as shown in Fig. 7) [24,41]. Secondly, the pyridinic nitrogen atoms could enhance the reversible specific capacities of the nitrogen-doped graphene sheets electrode compared with the graphene sheets electrode [20,26,42]. Thirdly, high nitrogen-doping level in nitrogen-doped graphene sheets could be beneficial to the high reversible capacity [12,25,42]. 4. Conclusions High-level nitrogen-doped graphene sheets have been successfully prepared by a new facile approach using the low-cost industrial material melamine as the nitrogen source. The atomic percentage of nitrogen in the nitrogen-doped graphene sheets is as high as 7.04 at.%. The high-level nitrogen-doped graphene sheets present a high reversible specific capacity of 1123 mAh g−1 at the current density of 50 mA g−1 and excellent rate capacity of about 241 mAh g−1 , even at an extremely high current density of 20 A g−1 . Compared with existing nitrogen-doped graphene, the asprepared high-level nitrogen-doped graphene sheets show more excellent electrochemical performance. The results demonstrate

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